Hook flange guard rails were originally designed to simplify the installation and maintenance of guard rails along trackwork. Unlike other guard rails, a hook flange guard rail is made up of a rolled steel rail section having one of its two flanges lowered to form a hook which fits beneath the base of an adjacent running rail. As shown in a 1946 Bethlehem Steel catalog entitled "Steel Products", page 218, such hook flange guard rails are held in place by placing the hook flange under the base of an adjacent running rail and wedging the opposite flange against shoulders provided on tie plates.
Anchoring the hook flange under the adjacent running rail utilizes the weight of the rolling stock to prevent overturning of the guard rail due to side thrust forces. This inherent resistance to overturning, which eliminates the need for using rail braces and clamps, has made the hook flange guard rail extremely popular within the industry. However, past practice has failed to provide means for adjusting the distance between a hook flange guard rail and its adjacent running rail. This absence of such lateral adjustment makes it necessary to replace hook flange guard rails at a greater frequency than state-of-the-art adjustable guard rails.